Digital video may be sent using frames of complete images, known as “I-frames”, separated by frames containing much less data (for bandwidth conservation purposes) known as “predictive frames” or “P-frames”, so called because such frames in effect extrapolate to a complete video frame using predictions based on the preceding I-frame.
A drawback of P-frames is that transmission errors in an I-frame (for example) can not only affect the decoding quality of the current frame but also can be propagated to ensuing P-frames, which are based on preceding frames. To alleviate this, the current rate of packet loss may be reported by the receiver to the sender, which can send I-frames at higher rates in response. Or, the identity of every lost packet (rather than a rate) can be reported, with the sender responding by immediately encoding and sending a repair frame that is referenced to a properly received older frame.